Electronic automatic pilots are well known to the art for controlling the actual heading of a vessel so as to maintain the vessel on a desired course or heading. In simplest form, such electronic automatic pilots include a heading error sensor which provides a heading error signal related to the angular deviation between the actual heading of the vessel and a desired heading that has been set into the heading error sensor by a navigator or operator of the vessel. A control surface angle sensor is also provided to furnish an output signal which is related to the angular deviation of a control surface of the vessel, such as the vessel's rudder, from the longitudinal axis of the vessel. The heading error signal and the output signal from the control surface angle sensor are differentially combined to develop an error signal which is used to control the position of the control surface so as to return the actual heading of the vessel to the desired heading.
A heading error sensor particularly suited for marine applications, and a marine electronic automatic pilot including such a heading error sensor, are disclosed and claimed in U.S. Pat. Nos. 3,906,641, and 3,946,691, respectively, which are both assigned to the assignee of the present invention. As described in these patents, the heading error sensor comprises a magnetic compass including a Hall-effect device whose output is proportional to the sine of the angular deviation between the actual magnetic heading and a desired magnetic heading of the vessel, and also comprises an electronic circuit responsive to the output signal from the Hall-effect device for providing a heading error signal which is related to the angular deviation, and the rate of angular deviation, of the vessel. The heading error signal and the output signal from the control surface angle sensor, which may also comprise a Hall-effect device, are supplied to respective inputs of a differential amplifier to develop the aforementioned error signal which is then coupled through a control surface driver to appropriately position the control surface or rudder of the vessel.
The conventional method of navigation, using such electronic automatic pilots, is by dead reckoning, in which the navigator determines the geographic coordinates (by latitude and longitude) of the present and desired geographic positions of the vessel, and computes therefrom a desired magnetic heading to the desired geographic position and sets that desired magnetic heading into the magnetic compass. In addition, the navigator computes the distance between the present and desired geographic positions, and computes therefrom the estimated time of arrival, given a predetermined vessel speed and also considering the effects of wind and current on the vessel.
The ability of an electronic automatic pilot to control the course of a vessel so as to arrive at a predetermined geographic position is limited, however, by a number of factors commonly encountered. As an example, the magnetic compass may not have been properly compensated for errors in the sensed magnetic heading of the vessel, such as heeling errors, northerly turning errors, and errors resulting from adjacent steel or other magnetic elements. Alos, the navigator may have made an error in computation of the desired magnetic heading, such as by failing to properly account for magnetic variation, or changes in magnetic variation, along the desired course. Likewise, the navigator may make an error in setting the magnetic compass to the desired magnetic heading, or may not be capable of precisely setting the desired magnetic heading (in degrees, minutes and seconds) due to the resolution capability of the magnetic compass. Most important, winds and waves which bear on the vessel, in directions generally transverse to the vessel's course, as well as transverse currents, cause the vessel to laterally drift from its desired course. Since the electronic automatic pilot is responsive only to heading error, it is incapable of correcting the vessel's course when the vessel experiences lateral drift but does not experience heading error.
When vessels are navigated by visual references such as are found on coastal and inland waters, frequent updates of vessel position using these visual references is possible, either by direct sighting or by radar. In cases where visual references are not available, as when the vessel is operating offshore, the navigator can determine vessel position by conventional celestial navigation, or by the use of electronic navigation systems. In all such cases, however, the navigator must manually make whatever course corrections are needed to insure that the vessel will arrive at the desired geographic position.
For general navigation purposes, it is not required that a vessel arrive at or return to a precise geographic position. There are many situations, however, in which such precision is required, for example, upon entering a harbor, and to locate fish traps, crab and lobster pots, fishing nets and set lines, oil well heads, fishing holes, mineral fields and so forth. To obtain such precision, navigators can use information obtained from an automated type of geographic position locator.
Such a geographic position locator may comprise part of an electric navigation system such as the well-known Loran-C system. The Loran-C system includes a plurality of Loran-C chains, each chain comprising a master transmitter and at least a pair of slave transmitters, all located at separate geographic positions. At predetermined group repetition intervals, the master transmitter in each chain transmits a pulse group having a predetermined format. Upon reception of the master pulse group, each slave transmitter in the chain transmits its own slave pulse group, again having a predetermined format, at a time which is very precisely controlled with respect to initiation of pulse transmission by the master transmitter. A Loran-C receiver on the vessel is used to detect and compare the master pulse group and a slave pulse group from one of the slave transmitters, in order to determine the time difference of arrival of such pulse groups at the Loran-C receiver. Due to the arrangement of the Loran-C system, the locus of all geographic positions where the observed time difference of arrival is constant comprises a fixed and invariable hyperbolic line of position. Therefore, the geographic position of the vessel along one such line of position may be ascertained by determining such a time difference. By determining the time difference of arrival of the master pulse group and a slave pulse group from another slave transmitter in the chain, a second line of position may be ascertained, with the actual geographic position of the vessel being represented by the intersection of the two lines of position. For further information relating to the construction and operation of the Loran-C system, reference should be made to Bowditch, American Practical Navigator, Pub. No. 9, Vol. 1, pp. 991-1002 (1977).
The Loran-C receivers that are presently available include a digital computer which is programmed, using information entered into the computer by the navigator, to compute the actual geographic position of the vessel. Further, these receivers have the capability of computing a desired track of the vessel between the actual geographic position of the vessel and a desired geographic position thereof, and for displaying to the navigator, in real time, the lateral distance that the vessel has drifted from the track, or, cross-track error, and the direction of such drift, that is, whether to the left or to the right of the track. Since geographic positions can be very accurately and precisely determined by the digital computer, the computer is also programmed to compute, and display, the time of arrival of the vessel at the desired geographic position. Using this information, the navigator is able to correct the actual course of the vessel and to thus arrive at the desired geographic position. Under normal conditions, a properly calibrated Loran-C receiver of this type is capable of computing geographic position to an accuracy of 1500 feet throughout a maximum range of 1200 nautical miles at day and 2500 nautical miles at night.
Although such Loran-C receivers are capable of very high accuracy and precision in determining geographic position, their use is subject to the disadvantage that the navigator must manually control the course of the vessel in accordance with the information provided by the Loran-C receiver. It has been proposed that the information obtained from a Loran type of receiver be used to develop a signal that would automatically correct the course of the vessel to maintain the vessel on the desired geographic track. However, such a proposal has not been subject to implementation in any practical form. Although the reliability of each Loran-C chain is very high (typically 95%), the ability of presently available Loran-C receivers to maintain a continuous determination of geographic position, and consequent determination of cross-track error, is limited by a number of factors, including inadequate signal-to-noise ratio in any received pulse groups, loss of one or more of the received pulse groups or distortion in the pulse groups as received, which results in the receiver not being able to effect accurate pulse group comparison. As another, more important example, the Loran-C receiver is incapable of determining the instantaneous heading of the vessel. As a result, any attempt to control the course of the vessel, using only the information from a Loran-C receiver, will result in continual short-term changes in the actual heading of the vessel. At best, the vessel's actual motion through the water would constantly zig-zag or oscillate about the desired track, and in the worst case, the vessel may be turned around 180.degree. from its desired heading.
There are certain other types of geographic position locators which would be useful in controlling the course of the vessel. To take fishing vessels as a particular example, it is oftentimes desirable for the vessel to track over water of constant depth, or, an isodepth, or to remain in water of equal temperature, or, an isotherm. In such cases, water depth and water temperature may be monitored by the use of a depth sounder and temperature sensor, respectively. The operator of the vessel may therefore correct the vessel's course in accordance with the information obtained from the depth sounder or temperature sensor. Although it is desirable to use this information to directly control the course of the vessel to follow the desired isodepth or isotherm, no practical means has been suggested for achieving this object, inasmuch as depth sounders and temperature sensors, like Loran-C receivers, are not capable of providing information as to the instantaneous heading of the vessel.
It is therefore an object of this invention to provide an apparatus which is capable of accurately and precisely controlling the course of a vessel in accordance with information obtained from a geographic position locator.
It is another object of this invention to provide an apparatus which is adapted to couple a geographic position locator to an electronic automatic pilot, and which functions to provide a course correction signal to the electronic automatic pilot, in response to the vessel position and/or vessel track error information from the geographical position locator, with the course correction signal being such so as to allow the electronic automatic pilot to maintain the vessel on a predetermined geographic track.
It is yet another object of this invention to provide such an apparatus which is capable of receiving and responding to information, in the form of analog or digital signals of various types, which are provided by a plurality of different types of geographic position locators, including Loran-C receivers, depth sounders, and temperature sensors.
It is a further object of this invention to provide such an apparatus which is capable either of using cross-track error information from a geographic position locator to develop a course correction signal for an electronic automatic pilot, or of computing a predetermined geographic track from information supplied to the apparatus by an operator of the vessel, and of correspondingly responding to geographic position information provided by a geographic position locator to develop a course correction signal in accordance with the thuscomputed, predetermined geographic track.
It is yet a further object of this invention to provide such an apparatus which includes a digital computer which is programmed to develop a digital cross-track error signal, using information supplied to the digital computer by one of a plurality of geographic position locators and by the operator.
It is still a further object of this invention to provide such an apparatus which includes a remote terminal, accessible by the operator, which is capable of interactive communications with the digital computer for the entering and displaying of information pertinent to the operation of the apparatus.
It is an object of this invention to provide such an apparatus which includes a digital computer and a remote terminal which can be easily and inexpensively implemented by the use of readily available microprocessor integrated circuit chips.
It is an additional object of this invention to provide, in such apparatus, an interface unit which is responsive to the digital cross-track error signal from the digital computer, and to other information from the digital computer, for selectively producing a course correction signal which is capable of combination with a heading error signal provided by a heading error sensor within an electronic automatic pilot.
It is another additional object of this invention to provide such an apparatus which is capable of terminating the production of a course correction signal, and of providing an alarm indication to the operator, upon detection of an alarm condition signified by alarm information transmitted from a geographic position locator such as a Loran-C receiver, or signified by instability in cross-track error information or geographic position information from the geographic position locator.